This invention relates to electric actuators, and more particularly to a high torque-to-weight ratio electric actuator for use in robotic systems, and including a resolver.
Robotic systems are used to reduce or eliminate the utilization of humans in certain industrial operations. Robotic systems are provided for the purposes of increasing productivity and of performing functions in hazardous environments, or locations where there is limited access, these situations either making manual operations hazardous or impossible. One application for robotic systems is in the repair or maintenance of nuclear reactor power plants, where the environment is radioactive, and human workmen may not occupy such environments, or may do so only for very limited periods of time, in order to limit the radiation exposure to the workmen.
In robotic arm systems, it is possible to place the actuators either away from the arm joints, driving them by such mechanisms as chains or belts or to place the actuators directly at each joint. The latter arrangement decreases the compliance of the arm, but reduces the arm load capability due to the added weight of the actuator on the arm. Remotely placed actuators, on the other hand, while reducing the weight of the arm, increase compliance and the inaccuracy of the arm's movement. Consequently, actuators located in the arms should not only be lightweight, but should provide high torque output. Hydraulic actuators previously used have provided high torque, but prior to the present invention, electric actuators were not used because they could not match the torque-to-weight ratios of the hydraulic actuators. Hydraulic systems are known to be more difficult to control, and where there is a vane type hydraulic system provided, it is not capable of continuous rotation. Hydraulic systems must have associated with them, in addition, such peripheral equipment as pumps and accumulators and there is the possibility of contamination of the hydraulic fluid, when used in hazardous environments.
Further, electric powered actuators for robotic applications, as in nuclear power plant servicing, must satisfy additional requirements. The position of the parts of the actuator must be able to be sensed, and where an electrical position sensor, such as a resolver, is used, it should be readily installed and replaced, in modular manner. Further, rigidity of the actuator and precision are required, as well as effective heat dissipation and braking.